The present invention relates to a method and system for controlling the air-fuel ratio for an internal combustion engine emission control system with a three-way catalyst, and more particularly to a system for controlling the air-fuel ratio to a predetermined value so as to effectively operate the three-way catalyst during such a deceleration which causes misfiring for preventing deviation of the air-fuel ratio caused by the misfiring.
Such a known system is a feedback control system, in which an oxygen sensor is provided to sense the oxygen content of the exhaust gases to generate an electrical signal as an indication of the air-fuel ratio of the burned air-fuel mixture. The control system controls the air-fuel ratio of the mixture to be fed to the engine in dependency on the signal from the oxygen sensor. However, if misfiring occurs in the engine during deceleration of the vehicle, a large amount of oxygen remains in the exhaust gas. The oxygen content of the exhaust gas then approximates that of exhaust gas which is produced when a lean mixture is induced and burned in the engine. Accordingly, the oxygen sensor generates a signal like a lean mixture detection, so that the control system operates to vary the mixture to a rich air-fuel ratio. Therefore, even if the actually induced mixture is a rich or proper air-fuel ratio, the mixture is excessively enriched by the control of the system. Thus, the fuel consumption of the engine deteriorates and unburned constituents of the exhaust gases increase.